To sell the public on the wisdom of turning the old Duke Diet and Fitness site into a pollution-controling wetland, the City Council told stormwater engineers they’d have to make a really good case.

They were right. When city engineers made their pitch to the InterNeighborhood Council in June, they faced questions and skepticism.

“You want to store all that nasty water right next to neighborhoods?” said Peter Katz of Old North Durham.

“What else might be feasible?” said Philip Azar of Trinity Park.

“Explain the science behind it,” said Chloe Palenchar of Old East Durham.

The engineers have another chance to explain and the public another chance to have its say Thursday night at City Hall.

Converting the 9.1-acre tract on West Trinity Avenue from a floodplain into a runoff-retaining and -filtering wetland could save taxpayers as much as $20 million to comply with water-quality standards for the Falls Lake watershed, according to a feasibility study.

Runoff from 485 acres of downtown and Trinity Park drains through the vacant Diet and Fitness property. A wetland occupying the full site, engineers estimate, could clear out as much nitrogen and phosphorus as 20 or more smaller installations, at a fraction of the cost.

However, building a 9.1-acre wetland means buying the site from Duke University and demolishing the Diet and Fitness building, with its swimming pool, gym and dining hall. Some residents have suggested turning the building into a neighborhood recreation center, and many are reluctant to give up on the idea.

Mike Shiflett of Northgate Park suggested it might be better, at least to start, to build a wetland about a mile downstream, in a 12-acre wedge of city-owned land between Club Boulevard and I-85 where South Ellerbe Creek, from the Diet and Fitness site, flows into the main stream.

That could help, too, said city engineer Lance Fontaine; but even with a wetland at Trinity Avenue, Durham is going to need perhaps 50 more pollution-control installations just for Ellerbe Creek – not to mention the other Durham streams feeding pollutants into reservoirs.

“This project,” Fontaine said, “is certainly not the end of it.”

That means there will be more projects in more neighborhoods and more convincing for the city to do.